Field of the Invention
The present invention relates to a semiconductor device used for controlling high-power electricity and, more particularly, to improvements in the insulation and heat dissipation scheme of a semiconductor device used for controlling high-power electricity.
Discussion of the Background
Semiconductor devices capable of controlling high-power electricity are used in industrial equipment in such fields as traffic control systems and motor control systems. For such semiconductor devices, it is necessary to ensure satisfactory electrical insulation and heat dissipation.
A semiconductor device for controlling high-power electricity in the background art may have a structure as shown in FIG. 7.
In FIG. 7, a semiconductor element 1 is bonded to an insulation substrate 3 by a solder 2, and the insulation substrate 3 is bonded to a base plate 6 made of copper by means of a solder 5 via a back electrode 32. The insulation substrate 3 is formed by bonding a front electrode 4, including front electrode elements 41, 42, and the back electrode 32 on the respective sides of an insulator plate 31. The semiconductor element 1 and the insulation substrate 3 are covered by an insulating cover 11, while a space below the insulating cover 11 is filled with a gel 9 injected through an opening 11a which is sealed by an epoxy resin 10. An emitter electrode of the semiconductor element 1 is connected via front electrode element 41 and conductor 7 to a main electrode 81, while a collector electrode is connected via an aluminum wire 14 and front electrode element 42 to a main electrode 82.
The semiconductor device is used by connecting lead wires from an external circuit to the main electrodes 81, 82 by means of nuts 12 and installing a heat sink (not shown) to the base plate 6 by means of screws and set holes 13. Heat generated from the semiconductor element 1 by electric current supplied from the external circuit is transferred to the heat sink via the insulation substrate 3 and the base plate 6. In order to decrease the thermal resistance between the semiconductor element and the heat sink, an entire surface of the insulation substrate 3 is secured onto the base plate 6 by the solder 5. A heat conductive grease is generally applied to the interface between the base plate 6 and the heat sink.
The semiconductor device of the background art of FIG. 7, however, has the following problems.
In a process of manufacturing the semiconductor device, the insulation substrate 3 and the base plate 6 are heated to a melting point of the solder 5 and are then cooled down. Since a thermal expansion coefficient the base plate 6, which is made of copper, is about four times larger than that of the insulator plate 31 that is made of ceramics, the base plate 6 and the insulation substrate 3 are subject to different degrees of expansion and contraction. As a result, the base plate 6 that is bonded to the insulation substrate 3 warps to become convex on the top surface during the cooling process. As the base plate 6 warps, a gap is generated between the base plate 6 and the heat sink. This increases the thermal resistance between the base plate 6 and the heat sink, thus leading to decreased efficiency of heat dissipation from the semiconductor element 1.
Also, when semiconductor element 1 is heated/cooled during soldering, or when the temperature around the semiconductor 1 is increased/decreased by turning on/off the power supply to the semiconductor device, the solder 5 interposed between the base plate 6 and the insulation substrate 3 that have different thermal expansion coefficients is subjected to a stress. This stress tends to generate cracks in the solder 5 or in the bonding interface thereof. When cracks are generated in the solder 5 or the interface thereof, the thermal resistance between the insulation substrate 3 and the base plate 6 increases, thus leading to a decrease in the efficiency of heat dissipation from the semiconductor element 1.
Moreover, when the heat sink is screwed onto the base plate 6, a bending stress is generated in the base plate 6 and the insulation substrate 3 because the mating surfaces are not completely flat. The bending stress becomes particularly significant when the base plate 6 and the insulation substrate 3 are deformed to warp. If the insulator plate 31 made of ceramics breaks due to the bending stress, the semiconductor element 1 experiences dielectric breakdown.
The present invention has been made to address the above-described and other problems, and an object of the present invention is to provide a novel semiconductor device that has a low thermal resistance between a semiconductor element and a heat sink and that is capable of restraining the occurrence of dielectric breakdown in the semiconductor element due to cracks in an insulator plate.
In order to achieve the objects described above, the semiconductor device of the present invention includes (a) a base plate, (b) an insulation substrate including an insulator plate with a front electrode and a back electrode bonded thereto and fixed onto the base plate by the back electrode, (c) a semiconductor element fixed onto the insulation substrate and the front electrode, (d) an insulating cover covering the semiconductor element, and (e) electrodes led from the semiconductor element to the outside of the insulating cover. The base substrate also has a through hole which is smaller than the back electrode and is larger than the insulator plate, and the insulation substrate is placed in the through hole and is fixed onto the back surface of the base plate by the periphery of the back electrode.
The semiconductor device of the present invention is used with a heat sink secured onto its base plate, similarly as in the background art. However, unlike the background art, the insulation substrate makes direct contact with the heat sink, without the base plate intervening therebetween. Consequently, heat generated in the semiconductor element is transmitted from the insulation substrate directly to the heat sink, resulting in reduced thermal resistance between the semiconductor element and the heat sink.
Also, because the insulation substrate makes contact with the base plate only on the periphery of the back electrode placed on the back thereof, deformation due to the difference in thermal expansion coefficients between the insulation substrate and the base plate hardly occurs.
Moreover, the bending stress generated in the insulator plate when fixing the heat sink onto the base plate by screwing is mitigated by the back electrode. As a result, isolation between the back electrode and the front electrode due to cracks in the insulator plate can be prevented.
In the semiconductor device of the present invention, it is preferable that the back electrode is thicker than the front electrode. With such a configuration, as the temperature of the semiconductor device rises, the insulation substrate deforms to become convex on the bottom side due to the difference in stress generated on the front electrode side and the back electrode side. Such a deformation of the insulation substrate on the bottom surface becoming convex improves the close contact between the insulation substrate and the heat sink. As a result, the thermal resistance between the semiconductor element and the heat sink decreases as the temperature of the semiconductor device rises, thus leading to improved efficiency of heat dissipation.
Further according to the present invention, various materials may be used to make the base plate since the base plate does not take part in the thermal resistance. The base plate is preferably made of, for example, a plastic material that would reduce the weight of the semiconductor device.
Also, the base plate and the insulation substrate may be made of the same material while molding the base plate and the insulating cover in an integral body. By molding the base plate and the insulating cover in an integral body, construction of the semiconductor device can be simplified and the manufacturing cost can be reduced.
The semiconductor device of the present invention may also be provided with a base plate including a plurality of through holes therein. The individual insulating substrates with semiconductor elements fixed thereon are secured in the plurality of through holes. With this configuration, heat generated by the plurality of semiconductor elements can be dissipated through a common heat sink.
Alternatively, the base plate may be divided into a plurality of segments so that through holes are formed by proper arrangement of the segments. This configuration simplifies the construction of the semiconductor device and reduces the manufacturing cost.